1. Field of the Invention
The present invention relates to novel aqueous dispersions based on functionalized silicone oils that are crosslinkable into elastomeric state by removal of water therefrom.
2. Description of the Prior Art
Crosslinkable aqueous dispersions of the above type are known to this art in which the base silicone oil having silanol endgroups is prepared by emulsion polymerization.
Thus, U.S. Pat. No. 2,891,920 describes a process for the emulsion polymerization of polydiorganosiloxanes, employing an acidic or basic catalyst, in the presence of anionic, cationic or nonionic surface-active agents. This '920 patent relates that the emulsions obtained are stable in storage and, after the addition of fillers, can be used to formulate paints which provide a continuous coating by removal of water therefrom.
U.S. Pat. No. 3,294,725 describes, in particular, the use of dodecylbenzenesulfonic acid to polymerize polydiorganosiloxanes in emulsion. This '725 patent relates that to produce stable emulsions it is desirable to adjust the pH of these emulsions to a value of approximately 7. It also describes that an elastomeric coating can be obtained from these neutralized emulsions, to which colloidal silica and a polyalkoxysilane have been added.
U.S. Pat. No. 3,360,491 is similar to U.S. Pat. No. 3,294,725, except that the dodecylbenzenesulfonic acid is replaced with lauryl hydrogen sulfate.
U.S. Pat. No. 3,355,406 describes, inter alia, a silicone latex free of inorganic filler, comprising:
(i) a colloidal aqueous suspension of an .alpha.,.omega.-di(hydroxy)polydiorganosiloxane;
(ii) a silsesquioxane resin in the form of a colloidal suspension; and
(iii) a catalyst selected from among inorganic acids, strong bases, dialkyltin diacylates and organic or inorganic peroxides.
U.S. Pat. No. 3,697,469 describes a particular process for the emulsion polymerization of polydiorganosiloxanes and indicates the possibility of adding colloidal silica and a tin salt to this emulsion in order to produce an elastomeric coating by water evaporation.
French Patent FR-A-2,110,358 describes a silicone emulsion having a pH of between 6.5 and 9, crosslinking into an electrically conductive elastomer after evaporation of water by incorporation of carbon black. The emulsion, which additionally contains a tin salt and a polyalkoxysilane, is not stable in storage and must be stored in two separate packages (two-component emulsion).
U.S. Pat. Nos. 4,221,688 and 4,244,849 and French Patent FR-A-2,463,163 describe silicone emulsions which are stable in storage and comprise:
(i) an anionically stabilized emulsion of an .alpha.,.omega.-di(hydoxy)polydiorganosiloxane polymer;
(ii) a siliceous filler;
(iii) a tin salt;
(iv) optionally, a nonreinforcing filler.
The siliceous filler may be a colloidal silica (U.S. Pat. No. 4,221,688), sodium silicate (U.S. Pat. No. 4,244,849) or an amorphous silica powder (FR-A-2,463,163).
U.S. Pat. No. 4,618,645 describes the preparation of an aqueous latex of polyorganosiloxane crosslinked by mixing a polydiorganosiloxane having silanol endgroups, a polyalkoxysilane and an anionic surfactant of the alkali metal sulfate or sulfonate type. The functionalized silicone resin or oil is then necessarily polymerized (in fact, polycondensed) by anionic catalysts until the desired molecular weight is attained, in the presence of sulfonic acid or hydrogen sulfate (according to the examples, for 5 hours at ambient temperature and at a pH below 5), and is then neutralized to pH above 7.
U.S. Pat. Nos. 4,608,412 and 4,618,642 describe the addition of polyalkoxysilanes to aqueous dispersions of organopolysiloxane oils having silanol endgroups.
As best as can be determined, the prior art aqueous silicone dispersions capable of crosslinking into elastomeric state by removal of water always have contained, as a base silicone oil having silanol endgroups, an oil polymerized in emulsion in the presence of an anionic catalyst, which preferably also serves as the surfactant.
However, these aqueous dispersions present a storage stability problem which is difficult to solve, since the best-known technical solutions involve storing the dispersions at a high pH. Such high pH can be incompatible with certain applications.